734 EVENING DISCOURSES. 
will light at about five times the distance ; but it will only light when one definite 
note is sounded by the arc. These are the two distinct advantages of tuning, 
namely, greater distance and syntony, or responding to only one definite note. 
For wireless telegraphy by means of Hertzian waves, based on my arc method, 
we require much higher frequencies in the shunt circuit. If we attempt to obtain 
this higher frequency from the ordinary are burning between solid carbons in air, 
we find that above a certain limit the oscillations will no longer take place. This 
is due to the fact that we are varying the current through the are at this higher fre- 
quency too bere for an increase in current to be accompanied by a decrease in 
potential difference. I have demonstrated that if I only vary the current through 
the ordinary current arc sufficiently rapidly, then an increase in current is accom- 
panied by a proportionate increase in the potential ditference, and the are behaves 
just like an ordinary resistance. If we work with very small current arcs we can 
obtain high-frequency musical arcs burning in air either between carbon or metal 
electrodes. 
In a paper read before the International Electrical Congress at St. Louis in 
1904 Mr. Poulsen showed that by placing the are in a flame it was possible to 
obtain higher frequencies than when the are was burning in air. Following this 
up Mr. Poulsen came to the conclusion that the best results were obtained when 
the are was burning in hydrogen, or a gas containing hydrogen; and he further 
added a magnetic field around the arc somewhat similar to that which has been 
previously used by Elihu Thomson. 
The are burning in coal gas in a powerful transverse magnetic field was used 
by Poulsen in his early experiments to produce the high-frequency current neces- 
sary for wireless telegraphy between Lyngby and Esbjerg in Denmark. This 
apparatus has been further improved, and is now employed by the Amalgamated 
Radio-Telegraph Company in their station at Cullercoats and the other stations 
that they are erecting. 
In both the arc and the spark methods of wireless telegraphy we employ a 
high-frequency alternating current in the aérial conductor. The essential difference 
between the two methods lies in the fact that with the spark method our alternat- 
ing current in the aérial conductor first increases to a maximum value and then 
dies away rapidly, making only a limited number of oscillations, whereas in the arc 
method the oscillations are maintained continuously of unvarying amplitude. 
With the are method we are further able to choose the number of consecutive 
oscillations which make up each signal sufficiently great to obtain the very best 
syntony. On the other hand, improvement in the arrangement and construction 
of the apparatus for the spark method has so increased the number of oscillations 
corresponding with each spark that it may be that we shall be able to obtain a 
sufficient number in each train to give as good syntony by this method as that 
obtained with the are method. 
The are method seems eminently suitable for very high speeds of working. 
As the oscillations are quite continuous, we can cut them up into groups to form 
the dots and dashes of the Morse alphabet, just as if we were working with a 
continuous current such as is used on land lines, so that there seems no reason 
why as high a speed of working should not be obtained from the are method of 
wireless telegraphy as is obtainable by automatic signalling on land lines ; for it is 
to be noted that the dot or shortest signal of the Morse alphabet, even at a speed 
of three or four hundred words per minute, will last long enough to consist of 
many hundreds of oscillations of the current in the aérial, so that there will be 
plenty of oscillations in the group forming the dot to give good syntony. 
Turning to the spark method for high working speeds, we find a difficulty in 
that the dot of the Morse alphabet must at least occupy the average time required 
to charge the condenser or aérial and produce one spark, and preferably sufficiently 
long for several. We are therefore obliged in the spark method to use a high 
rate of sparking for high-speed signalling. This difficulty has not become very 
serious with the present low speeds of sending. When we come to use consider- 
able amounts of power to transmit messages over long distances, and we also 
require a high speed of working, the practical difficulty in constructing apparatus 
suitable for sufficiently rapid sparking will become serious. 
. ee 
ley 
